In one type of fault tolerant system, for each port, one set of hardware, called the command hardware or “COM”, transmits and receives signals and another independent set of hardware, called the monitor hardware or “MON”, monitors the signals transmitted and received by the command hardware. For example, in one such system, a wrap-back mechanism couples the transmitter of the COM hardware to a receiver in the MON hardware. Also, in such a system, for each port, each received signal is delivered to two independent receivers: one in the monitor hardware and one in the command hardware. Thus, for each port, such systems require three receivers and one transmitter: one transmitter and one receiver in the command hardware and two receivers in the monitor hardware.
In some applications, it is especially desirable to implement the physical layer of such a COM/MON system using similar hardware for both the command hardware and the monitor hardware such that they use the same number of transmitters and receivers. However, doing so typically leads to waste. FIG. 1 illustrates the physical layer portion of a COM/MON system that is implemented using the similar hardware for both the command and monitor hardware. In the example shown in FIG. 1, a device 100 comprises two ports 105. The device 100 is implemented using two transceiver circuits, one of which (also referred to here as the “COM IC” 104) is used to implement the command hardware for each port 105 and the other of which (also referred to here as the “MON IC” 106) is used to implement the monitor hardware for each port 105. Each of the circuits 104 and 106 includes four transceivers 108, each of which includes a respective transmitter 110 and a respective receiver 112.
The one transmitter and one receiver needed to implement the COM portion for each port 105 are implemented using a single one of the transceivers 108 in the COM IC 104. The transmitter 110 and receiver 112 used to implement the COM portion for each port 105 are coupled to the transmit portion 114 and receive portion 116 portion, respectively, of the communication channel 118 to which that port 105 is coupled.
The two receivers needed to implement the MON portion for each port 105 are implemented using the receivers 112 of two of the transceivers 108 included in the MON IC 106. One of the receivers 112 is coupled to the transmitter 110 for the respective COM portion for that port 105 using a wrap-back link 120, and the other receiver 112 is coupled to the receive portion 116 of the communication channel 118 in order to receive data from the communication channel 118.
The design shown in FIG. 1 uses similar hardware to implement the physical layer for each of the COM and MON portions for each port 105. However, as shown in FIG. 1, for each port 105 of the device 100, one entire transceiver 108 in the COM IC 104 and two transmitters 110 in the MON IC 106 are wasted (where the wasted elements are marked with an “X”).